Synaptic produces a GPG error: "signatures couldn't be verified because the public key is not available". How to add missing PPA public keys ?

When attempting to install packages or perform updates with Synaptic or from command line, the following warning is displayed:

W: GPG error: http://ppa.launchpad.net intrepid Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY XXXXXXXXXXX

Solution:

Please follow the instructions here for adding the keys for the PPA's that you are using:
https://help.launchpad.net/Packaging/PPA#Adding%20a%20PPA%20to%20your%20Ubuntu%20repositories

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Example:
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On update if you get an error like this...

GPG error: http://ppa.launchpad.net jaunty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY D0D3C959DB2035A6

Then you have to add the missing GPG key:

Open a Terminal from the menu Applications → Accessories → Terminal and type or better copy and paste a row a time then press enter:

sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com D0D3C959DB2035A6

Then type: (when the system ask you a password give your user password, you will not see nothing when you type it, then press enter)

sudo apt-get update
sudo apt-get upgrade
sudo apt-get dist-upgrade

to update your system.